POSTNATAL CHANGES IN NA,K-ATPASE ISOFORM EXPRESSION IN RAT CARDIAC VENTRICLE - CONSERVATION OF BIPHASIC OUABAIN AFFINITY

The cardiac glycoside sensitivity of the rat heart changes during postnatal maturation and in response to certain pathological conditions. The Na,K-ATPase is thought to be the receptor for cardiac glycosides, and there are three isozymes of its catalytic (alpha) subunit with different cardiac glycoside affinities: alpha-1 (low affinity) and alpha-2 and alpha-3 (high affinity). We examined the developmental expression of the alpha-subunit isozymes in rat ventricular membrane preparations by immunoblotting with isozyme-specific antibodies. The alpha-1 isozyme was present throughout all stages of maturation. A developmental switch from alpha-3 to alpha-2 occurred between 14 and 21 days after birth. Measurements of [H-3]ouabain binding and inhibition of Na,K-ATPase activity indicated that alpha-2 and alpha-3 should make equivalent contributions to ion pump capacity; in both neonatal and adult preparations, ouabain interacted with a single class of high-affinity binding sites (K(D) = 15 or 40 nM, respectively; B(max) = 4-5 pmol/mg protein), and at low concentrations produced a similar degree of Na,K-ATPase inhibition (25%). The results indicate that the developmental difference in cardiac glycoside sensitivity cannot be explained by quantitative differences in the proportion of high-affinity isozymes of the Na,K-ATPase. The switch from alpha-3 to alpha-2 coincides with other major changes in cardiac electrophysiology and calcium metabolism.